In high-performance very large scale integration (VLSI) systems, such as, for example, a data transmission system, it is often desirable to provide a signal at a variable frequency, with the frequency of the signal being set by a digital control word. This frequency should be able to be set within a prescribed bandwidth of the spectrum with a desired level of accuracy. To generate the signal, DCO circuits are often employed.
A DCO is an electronic system for synthesizing a wide range of frequencies from a fixed timebase. DCOs have a number of advantages over their phase-locked loop (PLL)-based analog counterparts: they are more flexible and usually more stable, assuming the timebase is substantially stable. Furthermore, for most controlled oscillators, frequency gain is a compromise between conflicting design objectives. A high gain is beneficial for achieving a wide frequency range, with the tradeoff being that it introduces frequency jitter when the control signal is noisy. In principle, this gain tradeoff is not applicable to a DCO because the noise immunity of its control input is very high.
Despite the excellent noise immunity of its control input, it is widely recognized that the jitter performance of a conventional DCO remains substantially worse compared to the jitter performance of its analog counterparts. This jitter is due, at least in part, to the fact that in a DCO the oscillator runs at discrete periods, thus producing an output signal having discrete frequencies associated therewith as a function of a digital control word presented to the control input of the DCO. The DCO is only capable of generating frequencies that are less than half the timebase frequency, due to Nyquist's theorem. In many systems, a divided down output of the oscillator is used and the amount of jitter introduced by the DCO is multiplied by the divide down value, thereby exacerbating the jitter problem. Previous techniques aimed at reducing jitter have generally involved increasing a resolution of the DCO. This approach, however, significantly increases the complexity, power consumption and silicon area of the DCO and is therefore undesirable.
Accordingly, there exists a need for a DCO having enhanced control resolution which does not suffer from one or more of the problems exhibited by conventional DCOs.